The invention relates to a synchronous network and in particular to a synchronous network comprising network nodes having a clock stability monitoring circuit to detect an instability of a reference clock distributed within the synchronous network.
FIG. 1 shows a conventional synchronous network comprising a chain of network nodes N to receive a reference clock being generated by a primary reference clock (PRC) generator. The primary reference clock generator can be for example located in a central office of a mobile service operator. The last network node Nn of the chain of nodes shown in FIG. 1 can be connected to a base station of a mobile telephone network. The nodes N as shown in FIG. 1 can be for example formed by synchronous Ethernet devices comprising ports to exchange data with each other. The conventional practice for monitoring time and frequency accuracy in a synchronous network such as shown in FIG. 1 is to deploy a synchronization test equipment, i.e. a synchronization tester as shown in FIG. 1 which can be connected to the network node N to be investigated e.g. to the last network node Nn of the chain as shown in FIG. 1. Further, the synchronization test equipment has access to the primary reference clock PRC generated by the PRC generator, for example via a GPS connection. The synchronization test equipment uses the reference time source to measure the accuracy of the synchronization in an end-to-end manner. The network node Nn of the node chain within the synchronous network can be for example a cell site gateway to which a base station of a mobile telephone network is connected. The base station is provided with a reference frequency by the cell site gateway and can be used by the base station for example for maintaining an accurate carrier frequency. The base stations have to be aligned to each other, for example by means of a UTC (Universal Time Coordinated) provided by a GPS receiver. However, the provision of a GPS receiver is expensive and the network becomes unscalable. Further, there are security concerns since a GPS signal can be jammed. Accordingly, it is desirable to have a synchronous network which provides a distributed reference clock for all network nodes of a synchronous network and also for all nodes connected to such a synchronous network, for example for a base station as shown in FIG. 1, wherein the reference clock is generated by a central reference clock generator such as the primary reference clock generator shown in FIG. 1. The reference clock or reference frequency is transported through the chain of network nodes from the source, i.e. the PRC generator throughout all network nodes until the last network node is reached.
Because of faulty circuits or devices within the network node chain the transported frequency information can be disturbed so that the upstream network nodes don't get the right reference information. In a conventional system the transportable synchronization test equipment is used to compare the primary reference clock generated by the central source with the clock signal of the investigated network node as shown in FIG. 1. This conventional way of detecting an instability of a reference clock distributed within a synchronous network has several disadvantages. The first disadvantage is that the synchronization test equipment has to be connected physically to the network node to be investigated so that the synchronization tester has to be transported and connected to the respective network node. A further disadvantage is that the synchronization test equipment needs to have access to the primary reference clock, for example via a GPS receiver. However, such a GPS signal is not available at many locations where the synchronization test equipment can therefore not be used. A further disadvantage of the conventional way of detecting reference clock instability is that it only provides an end-to-end coverage between the investigated network node and the primary reference clock source. It is not visible what happens between the investigated network node and the primary reference clock source. For example, if the reference clock of the investigated network node to which the synchronization tester is connected deviates from the primary reference clock PRC it cannot be detected where in the intermediate chain of network nodes the deviation has been caused. Accordingly, the synchronization test equipment connected to an investigated network node such as network node Nn as shown in FIG. 1 cannot identify the location of a faulty network node causing a deviation of the reference clock at the investigated network node. For example, if the deviating reference clock is caused by a faulty circuit within network node N1 synchronization test equipment connected to the last network node Nn is not able to identify the location of the error. Consequently, the conventional synchronization test equipment as shown in FIG. 1 the synchronization test equipment has to be moved upward towards the upstream network nodes until the erroneous network node is found. Naturally, this is very cumbersome and time consuming. A further disadvantage is that the synchronization test equipment is very complex and expensive and needs a physical interface to be connected to an investigated network node of the synchronous network.
Accordingly, there is a need for a synchronous network where instability of a reference clock distributed within said synchronous network can be efficiently detected.